Advances in micromachining technology have given rise to a variety of micro-electromechanical systems (MEMS) including light modulators for low cost display applications. Such modulators provide high-resolution, high operating speeds (KHz frame rates), multiple gray scale levels, color adaptability, high contrast ratio, and compatibility with VLSI technology. One such modulator has been disclosed in U.S. Pat. Nos. 5,311,360 and 5,677,783 by Bloom et al. This modulator is a micromachined reflective phase grating. It includes a plurality of deformable elements in the form of beams suspended at both ends above a substrate. The deformable elements have a metallic layer that serves both as an electrode and as a reflective surface for incident light. The substrate is also reflective and contains a separate electrode. The deformable elements are disposed so that in an unactivated state light reflected off of the deformable elements has a phase relationship of m.lambda. with light reflected off of the substrate, where m is an integer and .lambda. is the wavelength of the incident light. Thus, in an unactivated state the modulator operates substantially like a mirror and the incident light beam is reflected. The deformable elements are activated by applying a voltage between the deformable elements and the substrate electrode, electrostatically drawing the deformable elements closer to the substrate. In a preferred embodiment, when the modulator is activated light reflected from the deformable elements has a phase relationship of p.lambda./2 with light reflected off of the substrate electrode, where p is an odd integer. In the activated state the modulator operates substantially as a reflective phase grating and the incident light beam is diffracted. Optical systems can intercept the diffracted light with output occurring only when the deformable elements are activated. For display applications, a number of deformable elements are grouped for simultaneous activation thereby defining a pixel, and arrays of such pixels are used to form an image. Furthermore, since gratings are inherently dispersive, this modulator can be used for color displays by appropriately altering the grating pitch.
Another MEMS light modulator has been disclosed in U.S. Pat. Nos. 5,233,459 and 5,784,189 by Bozler et al. This modulator comprises a plurality of moveable, reflective electrodes which are disposed on a transparent substrate containing a transparent electrode. In an unactivated state the electrodes are in a curled configuration adjacent to and unobstructive of a modulating region. In this unactivated state incident light passes through the substrate beneath the modulating region without reflection. The modulator is activated by applying a voltage between the moveable electrodes and the transparent substrate electrode, which electrostatically draws the moveable electrodes towards the substrate causing them to uncurl into a flat configuration over the modulating region. In the activated state the modulator behaves substantially like a mirror and the incident light is reflected from the modulating region. The modulator of Bozler et al is completely reflective in nature and stands distinct from the device disclosed by Bloom et al which is diffractive.
For many applications, particularly those requiring color, a diffractive light modulator is advantageous. One problem with the device disclosed by Bloom et al is device reliability. In a preferred embodiment the deformable elements contact the substrate in their activated state and have a tendency to stick. Additionally, the deformable elements tend to acquire a charge after a period of operation which electrostatically draws the elements toward the substrate and gives rise to a false activated state. Both of these problems are accentuated because the beams that make up the deformable elements have small restoring spring forces. It would be advantageous to produce a diffractive light modulator that operates substantially like the device of Bloom et al but has greater device reliability.